* Fix hardcoded `armhf` in pmb/aportgen/binutils.py
* Generate aports: `binutils-aarch64`, `musl-aarch64`, `gcc-aarch64`
* Distccd: Remember the cross-compiler architecture (currently armhf
or aarch64), that the current distccd is running as, and restart
distccd with the correct architecture, in case a different arch
is needed than what it is currently running as. (Depending on the
cross-compiler arch, the PATH variable gets adjusted before
starting distccd)
* Testcases: add aport generation for aarch64, add cross-compiling
to aarch64
* pmb/parse/arch.py: Add aarch64 to the mapping
